Vein Loading: The Role of the Symplast in Intercellular Transport of Carbohydrate between the Mesophyll and Minor Veins of Tobacco Leaves

Enzymatically separated leaf tissues of Nicotiana tabacum L., exhibiting good metabolic integrity, were used to evaluate the kinetics of sugar accumulation over the concentration range of 10 to 100 mm. Mesophyll cells exhibited Km values of 16 and 30 mm for glucose and sucrose, respectively; minor veins showed a reverse relationship, with Km values of 58 and 16 mm for glucose and sucrose, respectively. This would suggest that sucrose is preferentially absorbed by the minor vein net. Analysis of V(max) data indicates a reduction in the ability of isolated minor veins to accumulate substrate, implicating a symplastic rather than apoplastic route for intercellular transport. Competition studies demonstrate a common carrier for sucrose and glucose in both tissue types and suggest the presence of a "transport compartment," entry to which is regulated by a critical intracellular sucrose concentration.